1. Field of the Invention
The present invention is in the field of pumps for use in pumping oil from an oil well, and more specifically relates to a downhole sucker rod pump that is especially well adapted for pumping thick oils and oils containing particles of rock and sand. In the pump of the present invention the plunger is caused to rotate. This greatly extends the useful life of the pump by promoting even wear and by preventing galling of the plunger, by preventing sand from causing the plunger to become stuck in the barrel, and by helping to prevent the rod string from becoming unscrewed.
2. The Prior Art
Some oil wells initially produce a thin oil; as the well becomes depleted it produces a thicker oil that in many cases contains particles of sand and rock. Other oil wells produce thick sand-containing oil from the beginning.
Such wells are sometimes considered to be of marginal economic value because the product requires special treatment and because the oil is more difficult to produce. Part of the problem is that the sand in the oil is very hard on pumps and replacement of a pump is an expensive operation.
Specifically, the sand causes uneven wear of the plunger of the pump and of the pump barrel. Typically the wear-weakened parts fail and the pump must be pulled from the well and replaced. This expensive operation makes the well less desirable economically, and in many cases production is terminated, even though the well may still contain a sizable amount of usable oil. There are literally hundreds of such wells.
The motivation for the present invention is the idea that if a more robust pump could be devised, many of these uneconomical wells could be brought back into production.
The principle of distributing the wear on a piston more uniformly around its circumference by rotating the piston about the axis of the cylinder is not new. For example, in 1937 in U.S. Pat. No. 2,097,629, Lowrey described a piston having within the lower end a spiraled vane for producing a rotational motion causing the valve to wear uniformly. A somewhat similar piston was patented by Downing in 1894 in U.S. Pat. No. 518,490; uniformity of wear was not mentioned, and the perceived advantage was that the torque produced prevented the piston from becoming unscrewed from the pull rod, which clearly limited rotation of the piston.
What is believed to be new in the present invention is that the desired rotation of the piston is achieved without appreciably restricting the flow of oil through the plunger. This advantage is crucial for pumps intended for use with highly viscous oils and oils containing particles of sand and stone.
As can be seen in the drawings of Lowrey and of Downing, the vane structures they use are positioned at the lower end of the plunger in the oil intake port, and these vane structures severely restrict the flow of oil into the plunger, thereby reducing production, increasing the risk of plugging the flow path, and increasing the likelihood of "pounding" on the downstroke of the plunger.
In U.S. Pat. No. 627,039 Youroans describes a piston for a water pump. The piston includes segments that separate slightly on the downstroke to form passages for the water. Here again the flow path is severely restricted which would be especially disadvantageous if the pump were to be used for thick oil or oil containing sand or rock particles.
Fleming in U.S. Pat. No. 1,275,546 and Adams in U.S. Pat. No. 1,415,911 both show vanes located in the flow path near the lower end of the plunger and restricting the flow path, for the purpose of producing rotation to distribute wear more evenly.
The patents referred to above are all comparatively old, and it is unlikely that the pumps having these designs could survive long enough to be practical when operated at the pumping rates and rod string lengths commonly used today.
From the above discussion of the prior art it is clear that while the principle of rotating the plunger to distribute wear more evenly is well-known, no one has successfully applied that principle to the design of a plunger for a pump intended for use with thick oil and oil containing sand or rock particles, where the even distribution of wear is especially important but where restriction of the flow of oil through the piston must be minimized.